Geochemistry of Banded Iron Formations and their Host Rocks from the Central Eastern Desert of Egypt: Genetic and Tectonic Implications

Thirteen banded iron formations (BIF) occur intercalated with Neoproterozoic volcanosedimentary units in the Egyptian Central Eastern Desert (CED). These BIFs consist of layers of magnetite and hematite-rich layers alternating with quartz-rich layers containing garnet, epidote, ± calcite in the southern parts of CED, and chlorite ± stilpnomelane, and calcite in the north. Localized hydrothermal alteration manifested by secondary Ca-bearing minerals affected all BIFs. All BIFs and their host rocks were strongly deformed and metamorphosed under greenschist to epidote amphibolite facies conditions during the collisional stage of the Pan African orogeny.

Geochemically, CED BIFs have higher Fe/Si compared to Algoma, Superior, or Rapitan BIF types. All BIFs have rare earth element - Y patterns similar to modern day oceanic water, with a few samples displaying a weak positive Eu anomaly. All BIFs have high SiO₂/Al₂O₃ and Fe/Ti, and low Al/(Al+Fe+Mn), which suggest a hydrogenous origin with hydrothermal contributions and low detrital component. Factor analysis of geochemical data, geochemical trends, Ho/Y, and Pr/Yb values suggest deposition of Wadi El Dabbah BIF closest to the active arc and Um Nar (to the south) and Wadi Kareim (to the north) closest to submarine hydrothermal vents. Host metavolcanic and metavolcaniclastic rocks show chemical signatures indicative of an immature oceanic arc setting with MORB affinities for the southern bodies and back or fore arc basin affinities for the northern localities.

These results suggest that CED BIFs and their host rocks are non-glaciogenic, having formed in small sloped or terraced silled basins in the back and fore arc areas surrounding an immature island arc. Fe²⁺ and Si were provided from submarine hydrothermal exhalative vents. Volcanic eruptions filled the small silled basins with ash which imposed anoxic conditions leading to increased Fe⁺² and Si in solution as submarine hydrothermal activity continued. During periods of arc quiescence, increased biotic activity resulted in oxidation of Fe²⁺ and deposition of Fe-oxyhydroxide. Diagenesis formed fine-grained magnetite, whereas subsequent localized hydrothermal alteration related to serpentinization or magmatic intrusion, and regional metamorphism, formed porphyroblastic magnetite and specularite.